In a standard direct-injection diesel engine for heavy goods vehicles, i.e. a diesel engine with a cubic capacity between 0.5 and 4 liters per cylinder, the engine is controlled for a combustion at a maximum cylinder pressure amounting to about 180 bar at 22 bar BMEP. In such an engine, fuel is injected directly into the combustion chambers at about 30 crank angle degrees when the internal combustion engine is under maximum load. At maximum load, the injection is usually initiated 10–15 degrees before upper dead center and continues up to about 15–20 degrees after dead center depending on the operating point of the engine. A conventional internal combustion engine of this type is fitted with a turbo unit having a turbo map efficiency amounting to about 55–60%. The maximum charge pressure from the turbo unit amounts to about 330 kPa absolute pressure. Conventional diesel engines of the above-stated type have a thermal efficiency amounting to about 45–46% at maximum. By thermal efficiency is meant that share of the energy content of the fuel which is released during combustion which the engine is capable of converting into useful mechanical work.
In recent years, statutory requirements pertaining to emissions from diesel engines, especially relating to discharges of nitrogen oxide compounds and particles, have been tightened. The quantity of nitrogen oxides which is formed when fuel is combusted in a cylinder is dependent on the temperature and duration of combustion. Higher temperature leads to a greater share of the nitrogen in the air being converted into nitrogen oxides. One way of reducing the quantity of formed nitrogen oxide is to reduce the temperature at combustion. Reducing the temperature at combustion, however, creates problems. In certain operating conditions, the quantity of soot particles increases, which can result in an engine, for this reason, failing to win approval under prevailing emissions legislation. Moreover, the thermal efficiency of the internal combustion engine may diminish when the temperature falls. Nitrogen oxides formed during the combustion can be reduced, however, and hence re-converted into nitrogen by the after-treatment of exhaust gases in catalytic reaction chambers located in the exhaust pipe. The presence of catalytic reaction chambers raises, however, the exhaust-gas back-pressure. An increased exhaust-gas back-pressure causes a fall in the thermal efficiency of the internal combustion engine. Furthermore, the demands for reduced discharges of soot particles can necessitate the use of so-called particle traps, should the internal combustion engine, in certain operating points, generate excessive particle quantities, in order to satisfy prevailing emission requirements. Particle traps also give rise to increased exhaust back-pressure and hence lower thermal efficiency for the internal combustion engine.
One problem facing manufacturers of internal combustion engines on which statutory requirements are imposed with respect of maximally permitted emission levels of soot particles and nitrogen oxide compounds lies in the fact that the required permitted emission levels are constantly being lowered. Demands for reduced emission levels mean, firstly, that the engine cannot be optimized for low fuel consumption and, secondly, that emission-reducing peripheral equipment is called for which contributes to reduced thermal efficiency for the internal combustion engine.